The subject matter as disclosed herein relates to industrial controllers communicating among components by computer networks and in particular by Ethernet type networks.
Networks used for communication among industrial controllers differ from standard networks in that they must operate to communicate data reliably within predefined time limits. Often this is accomplished by additional communication protocols that reserve network bandwidth and schedule messages to prevent collisions and the like that can introduce unpredictable delay into network communications.
Many computer networks also incorporate protocols to repair the network in the event of network node failure. These protocols can take a relatively long time to reconnect the network (as much as 30 seconds) and thus are unacceptable for industrial control networks where the controlled process cannot be undirected during this period without disastrous consequences.
The risk of debilitating network failure in an industrial control can often be avoided using a redundant network topology, for example, where network nodes are connected in a ring with a supervisor. Normally the ring is opened at the supervisor node for all standard data and thus operates in a normal linear topology. The supervisor may send out test “telegram” or “beacon” frames in one direction on the ring which are received back at the supervisor in the other direction to indicate the integrity of the ring. If the ring is broken, such as by a node or media failure, the supervisor joins the ends of the ring to produce once again a continuous linear topology now separated by the failed component. Changes in the mode of operation of the supervisor from “separated” to “joined” may be transmitted to the other nodes using notification frames so that these nodes can rebuild their MAC address routing tables used to associate a port with a destination address.
The error detection time of such ring systems can be quite fast, limited principally by the transmission rate of the beacons (every several milliseconds). This rate defines the maximum time before which an error is detected and the ring may be reconfigured.